Based upon their pathogenicity, two strains of Friend erythroleukemia virus were isolated in the past: the FVA strain which causes anemia in susceptible mice and the FVP strain which causes polycythemia. Both FVA and FVP stocks contain two distinct components: a replication defective virus, the spleen focus-forming virus (SFFV), and an ecotropic murine leukemia virus which serves as a helper (F-MuLV). The defective SFFV was shown to induce an erythroproliferative disease in adult mice, whereas the F-MuLV helper was capable of inducing leukemia only in newborn mice. Studies were initiated to determine the genetic and molecular differences between the FVA and FVP strains of SFFV which may account for the differences in their biological properties. Presently, attempts are being made to clone a biologically active form of FVA DNA in E. coli using a plasmid as a vector.